Heavy and nutritional rocks?

Hi everyone!

Im here today trying to kill 2 birds with one stone (pun intended). I need to add some weight to these dwc pump boxes I’ve made, and would like to know if anything suitable will also leach nutrients to my water column.

I have ditched those loud ass air pumps for water pumps (like in aquariums, or ponds) that pump water from deep in the reservoir to a hose that sprays down in multiple spots above the water line. This is supposedly a good alternative to incorporating air to keep the dissolved oxygen levels up.

The system is working as it should, but I know as time passes and the roots get more massive, they will inevitably start to clog up my pumps. I have built boxes to hopefully help prevent that. These boxes have lots and lots of holes, then go into a mesh bag, and then into the water. Hopefully the majority or the roots will be kept out, and allow the water through helping the pump to work as it should.

The problem I foresee happening now is the action of pumping causes the boxes to float up just a little due to the pumps also drawing in water from the bottom. With the shallowness of my reservoir, this could cause a problem as the water level drops before refilling. I would like to add something to the box that would sink it back down. I could just secure it, but hopefully I can find something that will have more than just that benefit.

I have aragonite that I use in my fish tank, brick chips in my yard, and gravel is readily available too.

So my question is: “What is the best source for a heavy rocks that I could fill in (or put in a mesh bag) around the pump inside the box that would help keep it sunk, and provide me with some bonus nutrients as well?”

It would also become another added bonus and become another layer of protection from the few roots I still expect to see make it through the outer mesh bag, and then the holes in the box.

I appreciate your suggestions!